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UC-NRLF 


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GIFT  OF 


[Reprinted  from  SCIENCE,  N.  S.,  Vol.  XLIL,  No.  1079,  Pages  800-80S,  September  3,  1915] 


ARE  EECESSIVE  CHARACTERS  DUE  TO  LOSS? 


SINCE  the  presence-absence  theory  came  into 
vogue  it  has  become  quite  customary  to  re- 
gard recessive  characters  as  due  to  the  absence 
of  something  in  the  germ  plasm  on  which  the 
corresponding  dominant  character  depends. 
The  nomenclature  of  the  presence-absence 
theory  has  been  adopted  by  most  writers  on 
Mendelian  inheritance,  and  it  has  afforded  a 
useful  and  convenient  method  of  expressing 
gametic  formulas,  although,  as  Morgan  has 
shown,  there  are  cases  in  which  it  leads  to  in- 
consistent results.  While  it  is  often  recog- 
nized that  this  nomenclature  is  a  purely  sym- 
bolic scheme  of  indicating  how  certain  char- 
acters behave  in  inheritance,  the  habitual  em- 
ployment of  the  system  in  the  search  for  form- 
ulas which  will  designate  by  a  series  of  large 
and  small  letters  the  gametic  constitution  of 
the  organisms  one  is  dealing  with,  has  a  strong 
tendency  to  influence  one's  views  in  regard  to 
several  important  problems  of  heredity  and 
evolution.  I  can  not  but  think  that  the  opin- 
ions of  many  students  of  genetics  have  been 
unduly  influenced  by  their  formulas.  Form- 
ulas are  excellent  servants  but  bad  masters. 
Almost  involuntarily  a  certain  interpretation 
is  attached  to  their  symbolism  which  is  apt  to 
have  the  practical  effect  of  actual  belief  if  it 
does  not  succeed  in  producing  it. 

Since  the  establishment  of  Mendel's  law  and 
its  successful  employment  in  elucidating  many 
previously  enigmatical  phenomena  of  inherit- 
ance, heritable  variations  have  commonly 
come  to  be  considered  as  due  to  the  addition  or 
subtraction  of  discrete  units  of  germ  plasm, 
the  bearers  of  unit  characters.  Professor 
Bateson  in  his  "Problems  of  Genetics"  says 
in  regard  to  substantive  variations  that 

we  are  beginning  to  know  iri  what  such  variations 
consist.  These  changes  must  occur  either  by  the 
addition  or  loss  of  factors. 


And  further  on  he  makes  the  following  sig- 
nificant statement : 

Eecognition  of  the  distinction  between  dominant 
and  recessive  characters  has,  it  must  be  conceded, 
created  a  very  serious  obstacle  in  the  way  of  any 
rational  and  concrete  theory  of  evolution.  While 
variations  of  all  kinds  could  be  regarded  as  mani- 
festations of  some  mysterious  instability  of  organ- 
isms this  difficulty  did  not  occur  to  the  minds  of 
evolutionists.  To  most  of  those  who  have  taken 
part  in  genetic  analysis  it  has  become  a  permanent 
and  continual  obsession.  With  regard  to  the  origin 
of  recessive  variations,  there  is,  as  we  have  seen, 
no  special  difficulty.  They  are  negative  and  are 
due  to  absences,  but  as  soon  as  it  is  understood  that 
dominants  are  caused  by  an  addition  we  are  com- 
pletely at  a  loss  to  account  for  their  origin,  for  we 
can  not  surmise  any  source  from  which  they  have 
been  derived. 

In  his  more  recent  address  before  the  Brit- 
ish Association,  Bateson  not  only  interprets 
all  recessive  characters  as  due  to  loss,  but  sug- 
gests that  dominant  characters  may  have 
arisen  by  the  removal  of  inhibiting  factors, 
thereby  causing  a  "  release  "  of  the  characters 
which  previously  lay  latent  in  the  germ  plasm, 
and  producing  the  appearance  (but  only  the 
appearance)  of  new  variations.  He  says: 

In  spite  of  seeming  perversity  we  have  to  admit 
that  there  is  no  evolutionary  change  which  in  the 
present  state  of  our  knowledge  we  can  positively 
declare  to  be  not  due  to  loss. 

If  we  explain  not  only  the  actual  disappear- 
ance of  characters  as  caused  by  germinal  loss, 
but  the  appearance  of  new  characters  as  due  to 
the  loss  of  inhibitors  which  prevented  these 
characters  from  manifesting  themselves,  it  is 
theoretically  possible  to  consider  the  whole 
process  of  progressive  evolution  as  accom- 
plished by  the  sloughing  off  of  inhibiting  fac- 
tors. Such  a  doctrine  which  naturally  re- 
minds one  of  the  extravagancies  of  the  theory 


342603 


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SCIENCE 


of  emboitement  might  have  proved  quite  ac- 
ceptable to  Leibnitz,  Haller,  or  Bonnet,  but, 
unless  I  misunderstand  him,  Professor  Bate- 
son  has  presented  this  view  more  as  an  illus- 
tration of  the  bankruptcy  of  present  evolu- 
tionary theory  than  as  a  matter  of  serious 
conviction  of  his  own.  I  will  not  discuss  this 
interesting  speculation  further  than  to  observe 
that  any  interpretation  of  variation  which 
logically  leads  to  such  a  standpoint  naturally 
incurs  a  very  justifiable  suspicion  of  unsound- 
ness.  It  may  be  that  in  the  case  of  any  par- 
ticular variation  we  are  unable  to  positively 
declare  that  it  is  not  due  to  loss,  but  on  the 
other  hand  we  are  unable  to  positively  declare 
that  most  variations  are  due  to  loss.  I  think 
I  am  not  going  too  far  in  stating  that  a  ger- 
minal variation  due  to  loss  has  not  been 
proved  to  occur  in  any  single  case.  If  it  is 
legitimate  to  explain  the  appearance  of  new 
characters  as  due  to  the  removal  of  inhibitors, 
we  may  also  explain  the  apparent  loss  of  a 
character  as  due  to  the  advent  of  inhibitors. 
It  is  surely  justifiable  to  assume  that  inhibi- 
tors can  come  into  an  organism  somehow  if  we 
t  are  permitted  to  make  such  frequent  use  of 
their  disappearance  in  accounting  for  the' 
origin  of  new  variations.  The  .plain  fact  is 
that  we  know  practically  nothing  of  the 
i  changes  in  the  germ  plasm  which  we  postu- 
« \  late  as  the  causes  of  variability.  It  is  easy  to 
assume  the  existence  of  an  inhibitor  to  bring 
any  particular  variation  into  line  with  one's 
general  theory,  but  such  explanations  are 
purely  formal  and  therefore  of  little  scientific 
value. 

While  few  would  be  inclined  to  follow  Bate- 
son  in  his  rather  paradoxical  interpretation  of 
dominance,  the  doctrine  that  recessiveness  is 
due  to  loss  is  coming  to  be  quite  prevalent 
among  workers  in  genetics.  One  of  the  chief 
reasons  for  regarding  so  much  of  the  varia- 
tion that  has  arisen  among  domestic  animals 
as  caused  by  the  loss  of  factors  is  the  fact  that 
the  crossing  of  different  varieties  often  pro- 
duces a  reversion  toward  the  ancestral  type. 
If  we  regard  the  ancestor  of  our  races  of  do- 
mestic mice  for  instance  as  possessing  a  full 
complement  of  factors,  and  assume  that  the 
different  varieties  have  arisen  by  the  dropping 


out  of  one  or  more  factors  in  this  variety,  and 
one  or  more  other  factors  in  that  variety,  then 
when  these  varieties  are  crossed  the  hybrid 
may  possess  all  the  factors  of  the  original  an- 
cestor and  hence  show  a  reversion  to  type. 
On  the  basis  of  this  assumption  one  can  make 
out  gametic  formulas  for  the  different  vari- 
eties of  a  species,  test  them  by  breeding  ex- 
periments, and  thus  verify  their  correctness. 
Gametic  fomulas  obtained  in  this  way  doubt- 
less symbolize  a  truth  in  regard  to  the  germi- 
nal constitution  of  the  organisms  in  question. 
The  value  of  such  formulas  is  no  longer  a 
matter  of  doubt,  and  is  quite  independent  of 
the  various  interpretations  that  can  be  made 
concerning  the  nature  of  the  symbolism,  just 
as  chemical  formula?  are  of  value  quite  irre- 
spective of  the  various  theories  of  the  consti- 
tution of  atoms. 

Consider  the  origin  of  a  black  mouse  ac- 
cording to  the  presence-absence  hypothesis. 
We  may  explain  the  origin  of  a  black  mouse 
by  saying  that  it  is  caused  by  the  absence  of 
the  agouti  or  ticking  factor  that  breaks  up 
the  color  of  the  hair  into  bars.  Gray  is  there- 
fore black  plus  an  agouti  factor.  But  does  it 
' J  'follow  that  because  we  can  interpret  the  facts 
in  this  way,  and  interpret  them  consistently  so 
far  as  breeding  experiments  are  concerned, 
the  change  that  has  taken  place  in  the  germ 
plasm  that  produced**  a  black  mouse  was 
really  a  loss?  Such  a  change  is  frequently 
assumed  to  be  the  result  of  an  actual  loss  of 
a  little  discrete  unit  of  some  sort  in  the  germ 
plasm.  De  Vries  has  interpreted  recessiveness 
as  due  to  the  latency  or  loss  of  potency  of  pan- 
gens,  but  we  may  also  assume  that  the  germi- 
nal basis  of  the  character  in  question  has 
undergone  a  change  of  such  a  character  that 
without  becoming  inactive  it  ceases  to  func- 
tion in  its  usual  way.  The  agouti  factor 
(commonly  designated  by  G)  may  be  regarded 
as  dependent  on  a  part  of  the  germ  plasm,  a 
section  of  a  chromosome  possibly,  which  when 
present  causes  the  barring  of  color  in  the  hair. 
When  a  black  mouse  arises  we  may  suppose 
that  something  takes  place  in  G.  It  is  not 
necessarily  a  change  in  the  direction  of  either 
chemical  or  organic  simplicity  any  more  than 
it  is  necessarily  a  loss  of  substance.  The  fact 


SCIENCE 


that  the  modified  condition  is  recessive  to  G 
proves  nothing  whatsoever  in  regard  to  the 
nature  of  the  transformation  that  has  occurred 
in  the  germ  plasm.  Assuming  that  G  is  not 
actually  lost,  but  modified  into  another  kind 
of  substance  g,  the  recessiveness  of  g  may  be 
due  to  the  fact  that  its  activity  is  manifested 
in  a  different  way,  relative  slowness  of  its 
metabolism,  or  to  various  other  conceivable 
causes. 

There  is,  I  believe,  no  good  reason  for  con- 
sidering the  recessiveness  of  a  character  as 
due  to  the  relative  simplicity  of  its  germinal 
basis.  Many  variations  of  a  minus  character 
are  recessive,  but  there  are  numerous  excep- 
tions to  this  rule,  as  is  illustrated  by  the  domi- 
nance of  the  hornless  condition  in  cattle,  the 
short  tail  of  Manx  cats,  and  the  lack  of  beards 
in  certain  kinds  of  wheat.  Suppose  we  have 
two  allelomorphic  units  (assuming  for  the 
present  that  there  are  such  things  as  germinal 
units)  A  and  A',  one  of  which  tends  to  pro- 
duce a  relatively  simple  development  of  a  part 
and  the  other  a  relatively  complex  development 
of  a  particular  part  of  the  body.  The  one  A 
calls  forth,  say,  simple  horns,  the  other 
branched  horns.  A  and  A'  presumably  differ 
chemically,  and  the  development  of  the  part 
in  question  depends  not  upon  A  or  A'  alone, 
but  upon  how  these  agencies  affect  other  parts 
of  the  body  during  development.  Will  the 
simpler  substance  or  organic  unit  call  forth 
the  simpler  structure  in  the  adult  body?  In- 
asmuch as  the  development  of  any  organ  in- 
volves activities  in  which  a  larger  number  of 
elements  are  concerned  it  seems  not  at  all  im- 
probable that  the  simpler  substance  or  unit 
might  conspire  to  produce  the  more  complex 
organ.  Now  suppose  that  the  forked  horn 
proved  to  be  dominant  over  the  simple  horn. 
What  conclusions  would  we  be  entitled  to  draw 
from  this  fact  concerning  the  germinal  basis 
of  these  characters?  Obviously  none. 

Whether  we  interpret  a  variation  as  a  gain 
or  a  loss  is  in  most  cases  a  purely  arbitrary 
matter.  In  sugar  corn  there  is  a  loss  of  starch 
but  there  is  a  gain  of  sugar.  Does  sugar  corn 
therefore  represent  a  plus  or  a  minus  raria- 
tion?  Consider  the  familiar  cases  of  rose 
comb  and  pea  comb  in  poultry.  Both  of  these 


variations  are  dominant  over  the  primitive 
condition  of  single  comb.  Yet  both  breeds 
carry  the  basis  for  the  production  of  single 
comb  in  their  germ  plasm.  It  is  commonly 
assumed  that  both  conditions  represent  single 
comb  plus  something.  We  may  suppose  that 
in  a  certain  chromosome  a  change  has  taken 
place  which  results  in  the  development  of  rose 
comb.  This  change,  for  all  that  we  know, 
may  be  due  to  the  loss  or  impairment  of  a 
portion  of  germ  plasm,  or  it  may  be  due  to  a 
change  not  properly  describable  as  either  a  gain 
or  loss.  We  may  regard  rose  and  pea  comb  as 
more  or  less  pathological  deviations  based  on 
germinal  defect,  as  true  progressive  varia- 
tions, or  simply  as  normal  variations  neither 
progressive  nor  retrogressive.  So  far  as  com- 
plexity of  structure  is  concerned  it  may  be  a 
matter  of  dispute  whether  rose  comb,  pea 
comb,  or  single  comb  represents  the  higher 
grade  of  development. 

But,  it  may  be  asked,  are  not  color  varieties 
commonly  due  to  loss,  and  is  not  this  obviously 
the  case  with  albinism?  In  many  varieties 
there  has  certainly  been  a  loss  of  pigment,  but 
has  there  been  a  dropping  out  of  factors?  It 
by  no  means  follows.  The  factors  represented 
by  small  letters  in  our  color  formulas  are  by 
no  means  missing  entities.  They  are  changed 
so  that  they  occasion  a  diminished  production 
of  certain  pigments,  but  in  other  respects  they 
may  be  as  potent  as  before.  The  albino  does 
not  produce  pigment,  but  there  may  be  other 
substances  in  the  place  of  pigment  that  would 
distinguish  the  albino  as  a  positive  variation 
when  judged  by  other  standards.  The  animals 
whose  gametic  formulas  contain  a  number  of 
small  letters  are  not  necessarily  more  imper- 
fect or  perhaps  I  should  say  incomplete  than 
their  congeners  which  carry  a  large  number 
of  dominant  characters. 

Of  course  there  may  be  varieties  due  to 
losses  of  germinal  material.  Considering  the 
complex  mechanism  of  mitosis,  and  the  op- 
portunities afforded  for  the  loss  of  chromatin 
during  this  process,  such  variations  are  not 
improbable  a  priori.  But  there  is  not  the 
slightest  warrant  in  the  fact  of  recessiveness 
per  se  for  the  doctrine  that  all  recessive  varia- 
tions are  produced  by  this  method.  The  origin 


SCIENCE 


of  so-called  unit  characters  may  depend,  for 
the  most  part,  not  upon  germinal  loss  or  gain, 
but  simply  on  transformation.  Viewed  in  this 
simple  and  natural  way  the  appearance  of  a 
new  dominant  character  is  not  an  event  to  be 
marvelled  at.  Dominant  and  recessive  charac- 
ters not  improbably  owe  their  origin  to  much 
the  same  causes.  At  least  we  do  not  know  that 
they  do  not.  Concerning  the  real  causes  of 
variations  of  any  kind  we  know  very  little 
more  than  we  did  when  Darwin  commented 
on  our  profound  ignorance  of  this  subject.  It 
is  therefore  premature  to  pin  our  faith  to  any 
particular  theory  of  the  origin  of  variation  and 


especially  to  draw  far-reaching  conclusions  re- 
garding evolution  on  the  basis  of  such  an  in- 
terpretation. We  may  conceive  variability  as 
due  to  germinal  losses  or  gains  for  the  sake  of 
our  formulas,  and  there  may  be  little  harm  in 
so  doing  so  long  as  it  is  clearly  realized  that 
the  procedure  is  a  purely  arbitrary  and 
schematic  method  of  recording  certain  facts 
of  inheritance.  But  when  we  make  the  serious 
attempt  to  apply  the  conception  to  what  actu- 
ally takes  place  in  the  germ  plasm  we  en- 
counter a  fruitful  source  of  fallacies. 


S.  J.  HOLMES 


UNIVERSITY  OF  CALIFORNIA 


3426G3 


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NOV    9  1940 


